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Petrol (gasoline in the United States and Canada) is a petroleum-derived liquid mixture consisting primarily of hydrocarbons, used as fuel in internal combustion engines. The term gasoline is the common usage within the oil industry, even within companies that are not American. Often the term mogas (short for motor gasoline, for use in cars) is used to distinguish it from avgas, used in light aircraft.1 Chemical analysis and production
Petrol is produced in oil refineries. These days material that is simply separated from crude oil via distillation, called natural gasoline, will not meet the required specifications (in particular octane rating; see below) for modern engines, but these streams will form part of the blend.
The bulk of a typical petrol consists of hydrocarbons with between 5 and 12 carbon atoms per moleculeIn science, a molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. A molecule consists of multiple atoms joined by shared pairs of electrons in a covalent bond''. It may consist of atom.
The various refinery streams that are blended togther to make petrol all have different characteristics. Some important streams are:
- Reformate, produced in a catalytic reformer with a high octane and high aromaticIn chemistry, an aromatic molecule is one in which electrons are free to cycle around circular arrangements of atoms, which are alternately singly and doubly bonded to one another. More properly, these bonds may be seen as a hybrid of a single bond and as content, and very low olefins ( alkeneAn alkene is one of the three classes of unsaturated hydrocarbons that contain at least one carbon- carbon double bond and have the general molecular formula of CH (the other two being alkynes and arenes). The simplest alkene is CH, which has the common ns).
- Cat Cracked Gasoline or Cat Cracked Naphtha, produced from a catalytic cracker , with a moderate octane, high olefins ( alkeneAn alkene is one of the three classes of unsaturated hydrocarbons that contain at least one carbon- carbon double bond and have the general molecular formula of CH (the other two being alkynes and arenes). The simplest alkene is CH, which has the common n) content, and moderate aromatics level.
- Natural Gasoline (has very many names), directly from crude oil with low octane, low aromatics (depending on the crude oil), some naphthenes ( cycloalkaneCycloalkanes are chemical compounds with a single ring of carbons to which hydrogens are attached according to the formula CH. They are named analogously to their normal alkane counterpart of the same carbon count: cyclopropane, cyclobutane, cyclopentane,s) and zero olefins ( alkeneAn alkene is one of the three classes of unsaturated hydrocarbons that contain at least one carbon- carbon double bond and have the general molecular formula of CH (the other two being alkynes and arenes). The simplest alkene is CH, which has the common ns).
- Alkylate, produced in a Alkylation unit, with a high octane and which is pure paraffin ( alkaneAn alkane in organic chemistry is a type of hydrocarbon in which the molecule has the maximum possible number of hydrogen atoms and so has no double bonds (they are saturated). The generic formula for acyclic alkanes, also known as aliphatic hydrocarbons), mainly branched chains.
- Isomerate (various names) which is made by isomerising Natural Gasoline to increase its octane.
(The terms used here are not always the correct chemical terms, typically they are old fashioned, but they are the terms normally used within the industry. The exact terminology for these streams varies by oil company and by country.)
Overall a typical petrol is predominantly a mixture of paraffins ( alkanes), naphthenes ( cycloalkanes), aromatics and olefins ( alkenes). The exact ratios can depend on
- the oil refinery that makes the petrol, as not all refineries have the same set of processing units
- the crude oil used by the refinery on a particular day
- the grade of gasoline, in particular the octane
These days petrol in many countries has tight limits on aromatics in general, benzene in particular, and olefins ( alkene) content. This is increasing the demand for high octane pure paraffin ( alkane) components, such as Alkylate, and is forcing refineries to add processing units to reduce the benzene content.
Petrol can also contains some other organic compounds: such as organic ethers, (deliberately added) plus small levels of contaminants, in particular sulfur compounds such as disulphides and thiophenes . Some contaminants, in particular mercaptans and hydrogen sulphide must be removed because they cause corrosion in engines.
